In 1884, Hans Christian Gram, a Danish doctor working in Berlin, accidentally stumbled on a method which still forms the basis for the identification of bacteria. While examining lung tissue from patients who had died of pneumonia, he discovered that certain stains were preferentially taken up and retained by bacterial cells. Over the course of the next few years, Gram developed a staining procedure which divided almost all bacteria into two large groups: Gram + and Gram - (although some may be Gram variable).